A wireless network conforming to Long Term Evolution (LTE) standard is a radio technology that is eventually intended to provide all services in a packet switched (PS) domain. However, circuit switched voice calls (denoted voice calls for short in the following) are still supported by 2G and 3G networks on behalf of the LTE network, and a mobile terminal having a LTE-subscription and making or receiving a voice call is therefore redirected to the 2G or 3G network.
In such situation, when the mobile terminal is connected to the packet switched network LTE, a function known as Circuit Switched Fall Back (CSFB) function allows the LTE network to set up Mobile Originating (MO) voice calls using the Circuit Switched (CS) domain of the 2G or 3G network, for instance GSM EDGE radio access networks (GERAN) or Universal Terrestrial Radio Access Network (UTRAN) used by Universal Mobile Telecommunications Systems (UMTS).
The CSFB function, described e.g. in TS 23.272 (Release 12, V12.4.0), directs the LTE mobile terminal to the 2G or 3G network for call setup processes, and the call remains in the CS domain until it is completed.
A drawback with the current 2G and 3G network specifications relating to the CSFB is that there is a requirement to run authentication in the CS domain using the Authentication and Key Agreement (AKA) protocol relatively often when setting up CSFB calls. To run the AKA protocol adds significantly to the call setup time.
FIG. 1 is a simplified signaling sequence illustrating setup of a regular Mobile Originated CSFB (MO CSFB) call, summarized in a few steps. The mobile terminal (MT) 1 is first registered to the Mobile Management Entity (MME) 2 of the LTE network, as indicated at arrow A1. This registration comprises authentication, identity check and security key update. Next, indicated at arrow A2, the MT 1 sends an extended Non-Access Stratum (NAS) service request to the MME 2. The MME 2 informs the Mobile Switching Centre (MSC) 3 of the 2G or 3G network about a coming CSFB call (arrow A3). In response, the MSC 3 allocates resources for the call (arrow A4). Next, indicated by arrow A5, the MT 1 sends a call management (CM) service request to the MSC 3 for establishing the call. Finally, indicated by arrow A6, the MSC 3 sets up the MO CSFB call.